The present invention relates to a fuel assembly, and particularly to a fuel assembly which has a water rod and is suitable for use in a boiling water reactor.
A conventional assembly for installation in a boiling water reactor comprises a channel box having a quadrangular cylindrical form and a fuel bundle which is received in the channel box. This fuel bundle comprises an upper tie plate and lower tie plate which are respectively provided at the upper and lower ends of the channel box, a plurality of spacers which are axially arranged at given distances in the channel box, a plurality of fuel rods which are regularly arranged in a square lattice form and pass through the spacers, and a water rod serving as a neutron moderating rod.
In such a boiling water reactor, the phase change of a coolant takes place in a fuel assembly and serves to remove heat. Therefore, the coolant flowing in the upwardly direction of the fuel assembly has a lower density at its upper position. Gap water, which is a region of saturated water, is present around the circumference of the channel box so that a control rod or a tube equipped with a neutron detector can be inserted therein. In this way, the fuel assembly arrangement is such that coolant is unevenly distributed through the fuel assembly in the horizontal and vertical direction during operation. Light water serving as a coolant acts as a moderator for neutrons, and, in a conventional fuel assembly, the ratio of fuel to moderator, an important factor in determining nuclear properties, depends upon the position of the moderator in the fuel assembly. A water rod which is disposed at the center of the fuel assembly improves the nuclear properties, as well as improving the stability of a reactor core. However, the heterogeneity described above has more influence in the case of a fuel assembly where an attempt is made to increase the average enrichment thereof view to increasing the degree of burn-up, thereby to realize more effective use of uranium resources and a reduction in the powder cost of generating power.
Examples of fuel assemblies which have been proposed to solve the above-described problem include a fuel assembly in which the number of neutron moderating rods is increased, a fuel assembly as disclosed in Japanese Patent Laid-Open No. 65792/1984 in which a large-diameter moderating rod having a diameter that is greater than the side of a fuel lattice unit is disposed, and a fuel assembly as disclosed in Japanese Patent Laid-Open Nos. 40986/1975 and 178387/1984 in which a moderating rod having a square cross section is disposed